A wireless local access network (WLAN) is a data transmission system to provide location independent network access between computing devices by using radio waves rather than a cable infrastructure. Often, WLANs are implemented as the final link between existing wired network and a group of client computers, giving these users wireless access to the full resources and services of the corporate network across a building or campus setting.
The 802.11 specification as a standard for WLANs was ratified by the Institute of Electrical and Electronic Engineers (IEEE) in 1997. Like all of the IEEE 802 standards, 802.11 standards focus on the bottom two level of the International Organization for Standardization (ISO) model, the physical layer and the data link layer. The data link layer provides and functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. In the current context, this data link layer is further subdivided into Media Access Control (MAC) sublayer that manages interaction of devices with a shared medium. Above the MAC sublayer is the media-independent 1IEEE 802.2 Logical Link Control (LLC) sublayer that deals with addressing and multiplexing on multi-access media.
The physical layer defines all of the electrical and physical specifications for devices, and in particular the relationship between a device and a physical medium. The major functions and services of the physical layer are establishment and termination of a connection to a communications medium (e.g., wireless); contention resolution and flow control; and modulation, or conversion between representation of digital data into corresponding transmitted signals over a communication channel (e.g., radio link).
The 802.11 standards were implemented to provide reliable and secure wireless connectivity at high data rates. 802.11b and 802.11g standards use the 2.4 GHz band, operating in the United States under Part 15 of the FCC Rules and Regulations in the unlicensed Industrial, Scientific and Medical (ISM) bands. With the abundance of WLAN devices (e.g., access points, personal digital assistants (PDSs), laptop computers) in geographic proximity, interference is an increasing problem.
In addition, because of this choice of frequency band, 802.11b and 802.11g equipment could occasionally suffer interference from microwave ovens, sulfur lamps, wireless microphones, television broadcasts, or cordless telephones. Wireless personal area networks (PANs), such as Bluetooth devices, while operating in the same 2.4 GHz band, do not interfere with 802.11b and 802.11g in theory because they use a frequency hopping spread spectrum signaling method (FHSS) while 802.11b/g uses a direct sequence spread spectrum signaling method (DSSS). However, it should be appreciated that FHSS means that such devices should only occasionally collide on the same frequencies rather than not interfering at all.
Physical and MAC layer adaptation is critical for performance wireless networks to mitigate the effect of interference. Previously, it has been difficult for the radio to detect what the exactly source of channel degradation and therefore may not take the right adaptation actions. For example, most of current IEEE 802.11 radio decreases rate when it finds a packet loss. However, this adaptation may be wrong if the packet loss is cause by an instantaneously interference from a transmission of nearby node (i.e., collision).